There are many different forms of apparatuses for heating liquids, which involve the use of a heater member disposed in the container for the liquid and thus actually immersed in the liquid to be heated thereby. Thus, in one such apparatus, as disclosed in German laid-open application (DE-OS) No. 32 19 307, the apparatus is in the form of a water kettle comprising a vessel for receiving the liquid to be heated and, disposed in the vicinity of the bottom thereof, an electrical immersion-type heater having a mounting plate to which a heating element is secured. The mounting plate is secured by means of a single fixing device to the wall of the vessel, while a part of the active portion of the heating element is connected solely and directly to a heat-conducting portion of the fixing device. That arrangement provides a thermal bridge from the heating element to a thermally operated cut-out switch in the form of a bimetal switch which is disposed outside the vessel. The thermal bridge is formed by a round screw bolt with which the heating element is clamped to the wall of the vessel for containing the liquid. The arrangement is such that the switch switches off the heating element when a given limit temperature is exceeded.
However, in order for the thermally operated switch to respond to an excessive temperature, the thermal bridge formed by the screw bolt must comprise a material which is a good conductor of heat, for example copper, while the outer casing of the heating element, which is for example in the form of a jacket or casing tube, will generally comprise chrome nickel steel which has a substantially lower degree of thermal conductivity. As the casing tube of the heating element is additionally relatively thin, it also has a correspondingingly low heat capacity.
If the heating element overheats for any reason, for example becoming red-hot, then the amount of heat which flows away from the heating element by way of the relatively small contact area provided by the screw pin is so great that that area remains at a substantially lower temperature than the adjacent parts of the arrangement. As a result of that, either the normal operating temperature of the heating element must be set at a fairly low value, which has the consequence that an excessively long period of time is required in order to heat up the liquid to be heated in the vessel, or it is necessary to accept that critical temperatures will be exceeded somewhere along the length of the heating element.
In the above-described construction of a kettle with an immersion-type heater therein, the connection end portions of the heating element are disposed at substantially the same level as the thermal bridge to the bimetal switch. The connection end portions of the heating element must accordingly be kept inactive or inoperative in regard to heating action so that for example in the event of the liquid in the vessel boiling away or otherwise evaporating with the result that the apparatus runs dry, the connection end portions of the heating element do not suffer from overheating before the bimetal switch responds to the increased temperature. That in turn means that part of the length of the heating element is lost and useless in regard to producing a heating action, and is not available in regard to monitoring and checking overheating.